1. Field of the Invention
The present invention relates to a light source such as a fluorescent lamp or a halogen lamp which emits ultraviolet-radiation suppressed light, a coating agent for use in the light source, and a method of manufacturing the light source.
2. Description of the Related Art
A fading-suppressing fluorescent lamp (NU lamp) is conventionally used as an illumination light source for museum in which care must be taken to prevent fading of places such as a department store, an art gallery, and a products or exhibits.
The NU lamp has a structure in which two layers of a 5- to 10-.mu.m thick film containing titanium oxide (TiO.sub.2) having a grain size of 0.2 to 0.3 .mu.m and a 30- to 50-.mu.m thick film consisting of a fluorescent substance are sequentially formed on the inner surface of a glass tube. The titanium oxide film has an effect of absorbing ultraviolet radiation of 400 nm or less which is a cause of fading.
This NU lamp, however, has the following drawbacks as compared with a lamp of the same type not having a film containing titanium oxide.
(1) In order to obtain sufficient ultraviolet absorptive power, the film thickness of the film containing titanium oxide must be increased to increase the content of titanium oxide. As a result, a luminous flux of the NU lamp is reduced by 5% to 10% as compared with that of a normal lamp.
(2) Since titanium oxide absorbs ultraviolet radiation and visible light, especially blue light, the light color of the NU lamp is different from that of a normal lamp.
(3) In relation to the item (2), the color rendering property of the NU lamp is low.
(4) Since a high-temperature process at about 600.degree. C. must be performed twice in order to form a film having the two-layered structure described above, manufacturing steps of the NU lamp are complicated. In addition, the strength of a glass tube is reduced after the two high-temperature processes.
Published Unexamined Japanese Patent Application No. 61-110959 proposes an NU fluorescent lamp manufactured by forming a coating layer consisting of a solvent-soluble fluorine-containing polymer containing an ultraviolet absorbent on the outer surface of a glass tube. In this lamp, an organic compound such as a benzotriazole-based compound is used as the ultraviolet absorbent.
In this NU fluorescent lamp, however, a temperature for baking the fluorine-containing polymer on the outer surface of the glass tube is high, and the film thickness of the coating layer must be as large as about 10 to 30 .mu.m in order to obtain sufficient ultraviolet absorptive power.
Although a fluorescent lamp has been described above, even a halogen lamp or an HID (high-intensity discharge) lamp is often required to suppress ultraviolet radiation. As for these lamps, however, no suitable conventional means which can effectively suppress ultraviolet radiation and can withstand a high operation temperature of the lamp for a long time period have been provided.